Internet traffic has been growing exponentially and is expected to be more than 10 times that of voice traffic by the year 2005. This situation has triggered many research activities on wavelength division multiplexing (WDM) transmission and optical switching technologies. Considering the trend in IP network and the advance in optical technology, the next generation Internet will apparently depend on WDM network to transport expected huge amounts of Internet traffic.
Optical network based on WDM are evolving from present point-to-point transport links over add/drop multiplexers (ADM) and cross-connects for ring and mesh network, to network with higher reconfiguration speed. In the long term, optical packet switching seems to be a promising technology, but due to its complexity, much further research seems to be needed before it can be employed in a practical application.
The current use of a circuit switching mechanism is relatively simple to realize but requires a certain amount of time for channel establishment and release independent of the connection holding time. This overhead, mainly determined by the end-to-end signaling time, leads to poor channel utilization if connection holding time is very short. Research efforts to optimize network resources and protocols for IP traffic have focused on network architectures that can rapidly adapt to changes in traffic patterns as well as traffic loads.
Optical packet switching allows good bandwidth utilization, latency, and adaptability in an optical domain. At present, however, the optical packet switching is difficult to implement due to the lack of optical Random Access Memory (RAM) and other necessary signal processing capability.
Optical burst switching is spot lighted because it comprises IP over WDM circuit switching and pure optical packet switching with limited use of optical buffers. In optical burst switching technology, burst data can be transported without optical RAMs at intermediate nodes. In the OBS, a data burst cuts through intermediate nodes without being buffered, whereas in packet switching, a packet is stored and forwarded at each intermediate node. Compared to optical circuit switching, the OBS can achieve better bandwidth utilization because it allows statistical sharing of each wavelength among the flow of bursts that may otherwise consume several wavelengths. In addition, a burst will have a shorter end-to-end delay since the offset time used in the OBS is often much shorter than the time needed to set up a wavelength path in a wavelength routed network. However, the optical burst switching requires fast optical switching, which is still in a stage of research.
In order to implement the OBS network, there are a lot of challenging issues to be solved. The edge router, burst offset time management, and burst assembly mechanism are critical issues. In addition, the core router needs data burst and control header packet for scheduling, a protection and restoration mechanism, and a contention resolution scheme. The configuration and functions of the control plane in the OBS, including the control packet, are not yet standardized.
Since 1980, various electrical burst-switching techniques have been proposed: TAG (Tell-and-go), IBT (in-band-terminator) and RFD (reserve-a-fixed-duration), and so on. The TAG technique is similar to fast circuit switching. It transmits data bursts without an acknowledgement that bandwidth has been successfully reserved for the entire circuit. The IBT scheme reserves the bandwidth from the time the control packet is processed to the time the IBT is detected. In burst switching based on the RFD, bandwidth is reserved for a duration specified by each control packet; this eliminates signaling overhead and offers efficient bandwidth reservation.
JET (Just-Enough-Time) is a RFD-based burst switching protocol in the optical domain. It adopts two unique characteristics, namely, the use of offset time and delayed reservation. These features make JET and its variations more suitable for the OBS than OBS protocols based on the TAG or other one-way reservation schemes that do not adopt either or both of the features. The JET allows switching of data channels entirely in the optical domain by processing control packets in the electronic domain. A control packet precedes every data burst. Both the control packet and the corresponding data burst are separated by an offset time and are launched at a source node. The separate transmission and switching of data bursts and their headers help to facilitate the processing of headers and lower the optoelectronic processing capacity required at a core node. Moreover by assigning extra-offset time, the JET can be extended to support prioritized services in the optical domain.
The control packet contains information necessary for routing the data burst through the optical channel, as well as information on the length of the burst and the offset value. Another important characteristic of the JET is the delayed reservation. It reserves the bandwidth on each link just for the data burst duration. For example, let t1′ be the time when a first control packet arrives at a node after a control packet is processed and the bandwidth is reserved for a period from t1 (the time data burst arrives at a node) to t1+L1 (the data burst duration). This increases the bandwidth utilization and reduces the probability that a burst will be dropped. For example, in both cases shown in FIG. 1, namely t2>t1+L1(case 1) and t2<t1 (case 2), a second burst will not be dropped, provided that its length is shorter than that of t1−t2. However, when the second burst using the TAG arrives at t2′, it will be dropped because there is no buffer for it.
The functional model of an IP over a WDM network with the OBS is shown in FIG. 2. At an ingress node 10, edge routers determine a data burst size 34 and an offset time after considering the input IP traffic. Control packets 32, which contain information including an egress address, an offset time, a data burst size, and a QoS, go ahead on separate control wavelengths, and the main data burst 34 follows the control packet after a given offset time. These control packets are converted to electrical signals for processing at every intermediate node.
At the core node, bandwidth is reserved for the transmission time of the data burst. The elements that need to be monitored in traffic engineering are blocking probabilities, latency, and processing time. This information determines the optical path at the ingress node 10. At an egress node 20, the data burst 34 is deframed and disassembled into multiple IP packets in a rather simple manner. Burst reordering and retransmission is handled in the egress node if required.
Parameters, such as offset time, burst size, and quality of service (QoS) values, are essential in achieving an OBS network. These parameters are assigned in the ingress node of the OBS network.
In FIG. 3, the functions of the ingress node 10 are described in more details.
The first step to aggregate incoming burst IP traffic streams into a data burst is to assemble the burst data at a packet assembler 10a. The assembled data is then classified based on the priority of the IP traffic in a classifier C. The traffic can be further classified into congestion-controlled traffic and non-congestion-controlled traffic in Internet Protocol version 6(IPv6). In the case of the non-congestion-controlled traffic, the traffic is divided into eight classes based on the blocking rate. In IPv4, a Type-of-Service (TOS) field in an IP header allows one to choose from none to all of the following service types: low delay, high throughput, and high reliability. It also allows a priority selection from 0-7. Thus, considering both service types, eight or more classes are possible in this classification. Another consideration for classification is routing information. Routing information contains a specific combination of fiber (or port number) and wavelength. Assembling packets in separate queues provide more distinguishable differences in grades than using a unified class queue.
It is considered that there are two ways to assemble multiple IP packets into an optical data burst. A segmented method separates IP packets whenever necessary as shown in FIG. 4A, while a non-segmented method constructs earlier data bursts with idle data and puts IP packets in latter data bursts as shown in FIG. 4B. The segmented method offers high bandwidth utilization but requires complex hardware and a protocol system. The non-segmented method can be achieved more easily than the segmented method and reduces complexity but suffers from lower bandwidth utilization. In the OBS, the processing burden is heavy in an ingress and an egress node and the non-segmented method is better suited for assembling data bursts in the OBS.
In a burst-length decision step, the burst size is determined based on burst of input IP data (queueing length), QoS, and so on. In the OBS network, an offset time is generated in an offset time generator 10b on the basis of a burst length decision, and a lower class (or higher blocking rate) data burst affects a higher class (or lower blocking rate) data burst because higher class traffic is protected by adding an extra offset time to a base offset time. A control packet generator 10c generates the control packet, which contains information such as an offset time, a burst size, and a class number. Data in a buffer is scheduled and framed for transmission through a designated fiber.
A burst comprises of a burst header and a data burst. In the OBS, a data burst and its header are transmitted separately on different wavelengths with the burst header first. Each control packet includes information on switching, a burst size, an offset time, etc. Yijun Xiong gave an example of a data burst format in “Control Architecture in Optical Burst-Switched WDM Networks”, JSAC, VI, 18, No. 10, October 2000, but there has been no study on a control packet structure yet.
Meanwhile, at an edge node of the OBS network, edge routers assemble bursts by merging multiple IP packets. The data burst size should vary as little as possible, because a variation in large data burst size requires more extra-offset time for QoS which results in more delay. For reducing a variation of the data burst size, An Ge and Franco Callegati proposed a burst assembly algorithm using a timer-counter in “On Optical Burst Switching and Self-Similar Traffic”, IEEE, Comm Vol. 4, No. 3, March 2000.
However, this scheme resulted in low data burst utilization in a low offered load and huge variation in the burst size because the data burst size was not optimized for the input traffic. Moreover, a burst assembly algorithm based on a timer-counter may cause continuous blocking of data bursts in the core router as illustrated in FIG. 5. For example, suppose that a first control packet arrives at tCA1, and a second control packet arrives at tCA2 from an ingress node A and the first control packet arrives at tCB1, the second control packet arrives at tCB2 from ingress node B. And then, first data from the node A arrives at 1A1, second data from the node A arrives 1A2 after an offset time in an intermediate node X, and the first data from a node B arrives at 1B1, and the second data from the node B arrives at 1B2 after the offset time in the intermediate node X. Each control packet requests a bandwidth reservation in the nod X, and the node X can only honor the bandwidth request of the nodes A and B. If the ingress nodes A and B use a same timer period, i.e., Tperiod-a=Tperiod-b, a timer-counter-based scheme causes a high rate continuous blocking rate in a low offered load in reserving the bandwidth.
For the above-described reason, an OBS control packet structure based on MPLS is needed and a burst generation algorithm is necessary to generate high utilization data bursts and less variation in the burst size.